On Mon, Jul 22, 2019 at 3:44 PM Ondrej Mosnacek <omosnace@xxxxxxxxxx> wrote: > On Mon, Jul 22, 2019 at 1:35 PM NitinGote <nitin.r.gote@xxxxxxxxx> wrote: > > refcount_t type and corresponding API should be > > used instead of atomic_t when the variable is used as > > a reference counter. This allows to avoid accidental > > refcounter overflows that might lead to use-after-free > > situations. > > > > Signed-off-by: NitinGote <nitin.r.gote@xxxxxxxxx> > > Nack. > > The 'count' variable is not used as a reference counter here. It > tracks the number of entries in sidtab, which is a very specific > lookup table that can only grow (the count never decreases). I only > made it atomic because the variable is read outside of the sidtab's > spin lock and thus the reads and writes to it need to be guaranteed to > be atomic. The counter is only updated under the spin lock, so > insertions do not race with each other. Probably shouldn't even be atomic_t... quoting Documentation/atomic_t.txt: | SEMANTICS | --------- | | Non-RMW ops: | | The non-RMW ops are (typically) regular LOADs and STOREs and are canonically | implemented using READ_ONCE(), WRITE_ONCE(), smp_load_acquire() and | smp_store_release() respectively. Therefore, if you find yourself only using | the Non-RMW operations of atomic_t, you do not in fact need atomic_t at all | and are doing it wrong. So I think what you actually want here is a plain "int count", and then: - for unlocked reads, either READ_ONCE()+smp_rmb() or smp_load_acquire() - for writes, either smp_wmb()+WRITE_ONCE() or smp_store_release() smp_load_acquire() and smp_store_release() are probably the nicest here, since they are semantically clearer than smp_rmb()/smp_wmb().
